Telephone switching systems are linked by trunks, commonly referred to as voice trunks. Those trunks carry both signalling signals, e.g. which designate trunk seizure, destination digits, trunk release, etc., and voice signals.
Modern switching systems are being designed to deal with the considerable data that is to be transmitted between switching systems to provide new services to subscribers, such as calling station identification, roaming subscribers, etc., some of which require switching system processor to processor communication. In order to transmit the increased data between switching systems, specialized data links have been established between switching systems which are accessible to the switching system processors. These specialized and dedicated data links are much more efficient at transmitting this signalling data, since they are not required to carry analog or PCM signals, and can be optimized for digital data signals. Switching offices can make more efficient use of standard trunk circuits since the design of those trunk circuits can be restricted to and optimized for carrying analog or PCM voice signals, which are well known to have entirely different duty cycle and other characteristics than signalling data signals.
A well known standard protocol for transmitting "activity" signals via a separate data link between switching offices is referred to as SS7. Because typical messages sent using the SS7 protocol are large, e.g. 32 bytes in length, and because the data links transmit data between processors of separate switching offices, typical switching systems that can handle SS7 have a separate control shelf for interfacing the data links between the system processors. This is expensive, and requires reprogramming of the system processors.
When a data link is found to be faulty, all data carried by it is redirected to a different link. Once the faulty link is no longer faulty, traffic is usually returned to that link. It has been found difficult in prior systems to ensure that all messages that were, or were not yet transmitted on the faulty link could be transmitted on the replacement data link without any loss or repetition.